On 29 September 2014, Luuk van Dijk defended his MSc-thesis research entitled: "Influence of parasitic weeds on rice-weed competition". Luuk was supervised by Lammert Bastiaans and Aad van Ast. Niels Anten was the external examiner of the thesis.
Luuk's presentation can be found here.
Over the last decades rice (Oryza spp) became more important as a staple food crop for the African continent. In two decades, the locally produced rice in Africa doubled to almost 30 Mt in 2013. Still, the local production is insufficient to meet local demand and this particularly holds for Sub-Saharan Africa (SSA). Rice yields in SSA pertain to the lowest in the world. One of the major constraints accountable for these low yields are weeds. The parasitic weeds Striga asiatica and Rhamphicarpa fistulosa can cause devastating yield loses in SSA rice production. Recent field observations suggest that the presence of parasitic weeds influences the competitive relations between rice and non-parasitic (or ordinary) weeds. In the presence of S. asiatica the competitiveness of ordinary weeds was increased, whereas in the presence of R. fistulosa the ordinary weeds were further suppressed. Two pot experiments, carried out under greenhouse conditions, were used to study how the competitive relations between rice and the ordinary weed Mitracarpus villosus was affected by the presence of either S. asiatica or R. fistulosa. Plant dry biomass was used as a measure for the competitive effects. S. asiatica caused significant reductions in total pot biomass compared to pots with rice alone. Such a reduction was however not observed if next to rice also the M. villosus was present. This absence of a reduction in total pot biomass was not because the ordinary weed filled the gap that was created through the negative effect of the parasite on its rice host, rather rice biomass did not decrease in the rice-ordinary weed mixture. Emergence of S. asiatica in this mixture was lower, and this might be accountable for the minor effect of the parasite on the host. It is suggested that the root system of the ordinary weed might have disturbed the establishment of a connection between the host and the parasite, resulting in a reduced emergence of S. asiatica. R. fistulosa reduced rice biomass much more strongly than S. asiatica. In the rice-weed mixture, the ordinary weed was not able to profit from the reduced growth of the rice plant, as R. fistulosa grew fiercely and developed into a strong competitor. Consequently, the competitive ability of the ordinary weed, just as that of the rice plant, was strongly decreased. The experiments clearly show that the presence of a parasitic weed affects the growth and competitive relation between rice and ordinary weeds both directly and indirectly. The outcome of this complex multi-species interaction depends a lot on the parasitic weed species. R. fistulosa showed to be a damaging pest and utilized its ability to parasitize rice to gain a stronger competitiveness against the developed into the species dominating not only the rice, but also the ordinary weed. In case of S. asiatica, the ordinary weed reduced the infestation level of the parasitic weed and consequently the competitive relation between rice and the ordinary weed remained relatively undisturbed.
The full version of the thesis can be found here.